1. Field of the Invention
The present invention relates to reinforced films impervious to rain or other liquids or gases and useful for tarpaulins, boat and truck covers, lawn furniture, awnings, tenting, signs and the like. The films of this invention can attain significantly improved properties in terms of resistance to hydrostatic pressure, reduced water absorption and essentially no wicking of moisture. The present invention also relates to the manufacture of such films. In a preferred embodiment, the film includes a polyester open fabric and at least one polyester mat embedded within cured polyvinylchloride (PVC).
2. Description of the Related Art
Impervious, reinforced industrial membranes may consist of a reinforcing fabric sandwiched between two films of plastic. The fabric principally supplies strength, and the films make the membrane non-porous and impervious.
Higher tensile strength contributes to higher tear strength and puncture resistance. Good tear or breaking strength is necessary to prevent exacerbation of an accidental puncture Other key criteria are resistance to hydrostatic pressure and low wicking by the fabric.
In the past, primarily two kinds of fabric were available for the impervious, reinforced industrial membrane industry They were open, loose scrim (woven and non-woven) and very tightly woven, conventional, plain weave fabric.
The open scrim permitted very high adhesion between two films laminated on each side of the fabric because each film readily bonded to the other through the openings of the scrim. Because of this high degree of adhesion between the films, little or no adhesion was required between the film and the yarn of the fabric. The yarns could slide through the membrane very easily to "rope up" and provide a relatively high tear strength. However, because of the few or small number of yarns used to achieve the openness of the weave, the tensile strength was low. Moreover, due to the nature of the open scrim, it was difficult to use liquid coating processes to create two films on either side of these fabrics. Instead, membranes were made by laminating a scrim between two previously prepared films. For example, in making prior art membranes using open weave scrim fabrics, two PVC films were first created by extrusion, cured and then laminated to each side of a polyester scrim, perhaps with an adhesive between them. Such a lamination technique did not create a single, unitary film, and if not completely successful, resulted in a membrane that delaminated easily when stressed. Further, if any bit of delamination occurred, the yarns of the fabric tended to wick moisture into the membrane Wicking (allowing moisture to seep into and along the yarns at the edges, cracks or openings) exacerbates the problem of delamination.
An alternative prior art method for making a non-porous membrane using an open-weave fabric is the rotary screen method. In this method, a thick paste of very viscous thermoplastic polymer, having for example a viscosity on the order of 50,000 centipoise ("cps"), is placed inside a rotating cylinder made of a screen. A press or scraper extrudes the paste out of the cylinder where it is transferred to a fabric. If two such rotating cylinders are placed on each side of an open weave fabric, a film can be formed on both sides of it. However, the equipment for this process is expensive and the adhesion of the film to the fabric is not at a maximum because the thick paste does not fully impregnate the yarns of the fabric.
Tightly woven conventional weaves of fabric were also used in impervious, reinforced industrial membranes in the past for high tensile applications. Because there were essentially no openings in these fabrics, liquid coating processes could be used with them to create films on each side of the fabric. Because of the tight weave, however, it was impossible to create a single unitary film, and there was no adhesion of the film on one side to the film on the other. Strength therefore depended on the adhesion of the film to the yarn. This film-to-yarn adhesion, along with the tight weave, would tend to lock the yarns in place and keep them from sliding, significantly reducing the tear strength of the fabric. That is, the yarns were not able to work together (i.e., "rope-up" ) to resist the tearing force, but were torn individually.
Some attempts to manufacture an improved fabric have addressed the fabric weave. For instance, U.S. Pat. No. 3,250,662 relates to a modified weave design that was developed in the early 1960s. This fabric utilizes a warp having yarns of two different denier sizes arranged so that two of the smaller denier yarns alternated with the single yarns of the larger denier.
In a further weave design, the fill yarns were laid only on one side of the warp yarn rather than having them interlock between the warp yarn. Because of the nature of the equipment utilized in the late 1960's to provide the new weave design, the fill yarn was inserted in an overlapping crisscross pattern at a five-degree bias angle to the warp yarn.
In another prior art weave design, the fill yarns were inserted parallel to each other and perpendicular to the warp. This weave came to be known in the industry as weft inserted warp knit (WIWK) fabric. This weave gave better tear and tensile strength, but can be further improved by use of the present invention.
Accordingly, it is one object of the present invention to manufacture a non-porous, reinforced film, impervious to liquids or gases, that has the nature of a unitary film and is less susceptible to delamination and otherwise has improved properties.
It is another objection of the present invention to provide an impervious, reinforced, unitary film including an open fabric, sometimes referred to as a grid, web or scrim, provided with at least one light weight, porous mat and impregnated with plastic. The resulting film has a high resistance to hydrostatic pressure and is resistant to water absorption, i.e., is "low-wicking". Preferably, it is essentially non-wicking.
It is still another object of the instant invention to provide a process and apparatus whereby the above-described film may be made using an uncomplicated arrangement of machinery.
These and other objects that will become apparent and may be better understood by the detailed description provided below.